It has been common to prepare copper and chrome-arsenic compositions for use as timber preservatives from chromic acid, copper oxide and arsenic acid starting materials. In such a process, the arsenic acid may be derived from arsenic trioxide, but such arsenic trioxide must be particularly pure, otherwise the impurities therein would be carried through the process into the arsenic acid solution.
When such contaminated arsenic acid is used in the manufacture of CCA, the impurities become insolubilized in the CCA wood treating solution. The resulting precipitate not only represents an economic loss, but it may block pumps, valves and other portions of the handling and treating equipment. In addition, because of the chrome and arsenic content of CCA wood preservative, these precipitates represent a significant environmental hazard.
Although many acid-soluble substances such as calcium, aluminum, etc., are undesirable, the major problem-causing precipitate is acid solubilized iron.
As presently formulated, CCA preservatives cannot use copper metal as a raw material, because the copper metal would reduce the chromic acid causing total precipitation of the CCA. Since copper oxide and other suitable copper forms of raw materials for CCA manufacture, such as copper carbonate, are manufactured from copper metal, they always command a premium price above the base metal. Such prices, traditionally, are 50% to 100% higher than the base copper metal price.
Processes have been invented that allow the use of copper metal directly in the oxidizing arsenic acid process, but, unlike the present invention, all of these processes, because of their acidic nature, require a very pure form of copper. Any iron or other acid soluble impurity present in the copper or arsenic starting raw materials will be found in the copper arsenate formed by these processes and carried by it into the CCA causing precipitation.
During the past twenty years, there have been many occasions when suitable arsenic trioxide for arsenic acid manufacturing was in very short supply, either it was not available or available only at exorbitant prices. Such supply and price difficulties have applied equally to suitable copper and copper oxide materials.